The decision comes more than a week after the track suspended races at Santa Anita indefinitely after 21 horses were euthanized after sustaining injuries while training or racing at the track within two months.

“What has happened at Santa Anita over the last few weeks is beyond heartbreaking,” Belinda Stronach, president of The Stronach Group, which owns the track, said in the letter.

Princess Lili B, a 3-year-old racehorse training for her third career start, was euthanized at the Santa Anita racetrack on Thursday morning after breaking its front legs at the end of a half-mile workout, according to The Paulick Report, a horse racing news site.

David Bernstein, Princess Lili B’s owner and trainer, told KTLA-TV in Los Angeles that the racehorse was healthy before the injury.

“She was always very sound, and we’ve never had a problem with her,” Bernstein said in an interview with the news station. “We didn’t have to train her on any medication. She’s just a lovely filly to be around.”

Stronach on Thursday announced a list of changes to racetrack regulations that are likely to dramatically affect professional riders and their horses at Santa Anita.

“We have arrived at a watershed moment,” she added, before detailing a “complete revision” to the racetrack’s medication policy and other regulations.

Along with banning race-day medication, Lasix, a performance-enhancing medication, is also banned at the track. According to Inverse.com, Lasix prevents a horse’s lungs from spontaneously bleeding while running at high speeds. It is banned in other countries but used commonly with racehorses in the U.S.

Increasing the time required for horses to be on-site prior to a race.

A substantial investment by The Stronach Group in diagnostic equipment to aid in the early detection of pre-existing conditions.

Horses in training are only allowed therapeutic medication with a qualified veterinary diagnosis.

Earlier this month, Santa Anita officials ordered additional testing of the park’s one-mile main track after a 4-year-old filly had to be euthanized after an injury while training on the track. The Stronach Group has also completed ground radar testing on the track, which deemed it “one hundred percent ready” for use.

The nearly two dozen deaths at the track have marred the reputation of a racetrack that was once considered among the safest in the sport.

Tim Ritvo, the chief operating officer of The Stronach Group, told The Associated Press that the recent heavy rains in California may have affected the track and the horses’ health.

According to the park, 16 of the horses that died sustained injuries while racing or training on Santa Anita’s dirt track. Five died after racing on the track.

“We think that [rain] could definitely contribute, even though our experts are telling us not,” Ritvo told AP. “The tracks out here are built not for weather like that.”